The mitochondria are normal structures located outside the nucleus in the cell's cytoplasm. Each mitochondrion has a chromosome made of DNA that is quite different from the better known chromosomes in the nucleus. The mitochondrial chromosome is much smaller; it is round (whereas the chromosomes in the nucleus are normally shaped like rods); there are many copies of the mitochondrial chromosome in every cell; and no matter whether we are male or female, we inherit our mitochondrial chromosome from our mother.

All mitochondrial diseases were entirely enigmatic before it was discovered that they were due to mutations not in regular
chromosomes but in the chromosome of mitochondria.